Top foreign policy makers in the Carter administration reluctantly have concluded that Iran's exiled Moslem leader, Ayatollah Ruhollah Khomeini, now holds the key to the building of an anti-communist government around Prime Minister Shahpour Bakhtiar, U.S. officials report.

That assessment represents another large stride away from an administration policy on Iran that only a month ago could still be described as "the shah or chaos."

The shah's departure last week appears to have pushed the administration to seek a middle course after all. The administration's chief objective now is to get Khomeini, Bakhtiar, the Iranian military and others to agree on a political formula that will restore calm and keep non-communist forces in control of Iran, a senior State Department official told reporters yesterday.

Responding to questions, the official for the first time conceded "indications that there may be neutralist-type tendencies" in the power-sharing arrangement which could ultimately succeed the authoritarian and solidly pro-American government of the shah.

But he said the administration was encouraged because all of the main political forces "have said publicly and privately that they do not want a communist nation." Repeating pledges made by President Carter and Secretary of State Cyrus R. Vance not to intervene in Iranian affairs, he said this "consensus" made the administration "hopeful" that Bakhtiar's government could survive.

The official, who asked to remain anonymous, speaks authoritatively for the administration on foreign policy and appeared to be underscoring several points on which administration briefings have been ambiguous.

"We do not consider the Bakhtiar government to be a transitional government," he said. "We support the Bakhtiar government."

Asked directly if one measure of that support involves trying to get Khomeini to put off his promised return to Iran to confront Bakhtiar in a showdown in the streets, the official did not respond directly, saying "Those decisions are decisions he will have to make himself."

President Carter last week publicly asked Khomeini, who lives in exile near Paris, to give the Bakhtiar government "a chance to succeed."

Among points made by the official:

The administration has had no contact with Shah Mohammed Reza Pahlavi since he left Tehran on Tuesday for Aswan, Egypt. "He is on vacation," and the administration has been so busy dealing with "day to day" problems confronting the Bakhtiar government that there has not been a chance for contact, the official said.

Iran's top generals have been meeting daily as a "security council" and discussing the military's role in getting oil fields and communications working again after order is restored. "They are being supportive of the Bakhtiar government."

The administration will not significantly increase the U.S. military presence in the Persian Gulf and Indian Ocean region as a result of the sharp reduction of Iran's regional security role by the change in government.

"The problems are political problems," he said, but he added that other moderate countries will have to play "an increased role" in assuring the region's security now and may therefore require more U.S. military assistance. He did not list any of the other countries.

The official, confirming a report in yesterday's Washington Post that Crown Prince Fahd of Saudi Arabia is expected to pay an official visit to Washington around the first of March, indicated that the administration would press the Saudis then to offer open support for efforts by Egypt's President Anwar Sadat to sign a peace treaty with Israel.

The administration's grudging tilt toward conceding Khomeini's supporters an open political role in return for their acceptance of Bakhtiar became apparent last week in a series of appearances on Capitol Hill by administration officials.

"Bakhtiar is standing on a banana peel," Sen. Frank Church (D-Idaho) said after a Senate Foreign Relations Committee hearing on Iran Friday.

"The diplomatic effort now" is to get the Khomeini forces to reach agreement with Bakhtiar, Rep. Lee H. Hamilton (D-Ind.) said after hearing administration witnesses for two days. "Without it, Bakhtiar won't survive."

Asked yesterday for his assessment, the senior State Department official produced a reading that was not as starkly stated but which pointed in the same direction.

"It is important that the Bakhtiar government continue the discussions they are having with the various other political forces including the Khomeini people to see if they can broaden the support for the constitutional government that exists," he said.

Other officials said that the United States has played an important role in getting representatives of Khomeini, Bakhtiar, the Iranian military and the National Front political group to open talks in Tehran "and work toward a negotiated solution."

The senior official described these talks as a "dialogue among key non-communist political forces in the country" about reestablishing stability.He described Khomeini as "a man who does have very substantial political power," but said that the administration still has not established direct contacts with the ayatollah.

U.S. contacts with Khomeini aides in Tehran, France and Washington are continuing, he said. At another point he spoke of "political people who have been affiliated with Khomeini but who are political figures in their own right" as potential Bakhtiar supporters.

Similar phrases in the past have been used by U.S. officials who felt that an effort to neutralize Khomeini could be mounted by splitting National Front politicians and more moderate Moslem leaders away from the ayatollah. It was not clear from the official's remarks yesterday if that hope remains alive.

The official accused the Soviet Union of being "clearly unhelpful" in broadcasting attacks on U.S. policy in Iran from clandestine radios located on Soviet territory.

The Russians, who yesterday offered praise for Khomeini, have adopted a stance of "not choosing sides and keeping their options open," the official said, adding that the Moscow-backed Tudeh Party of Iran "has played a role, but a limited role" in the protests that drove the shan into exile.